Device for determining the size distribution of aerosol particles

ABSTRACT

The invention relates to a device ( 30 ) for determining the size distribution of aerosol particles from a flow ( 11 ), the device ( 30 ) comprising an electric mobility analyzer ( 10 ) and an impactor ( 20 ), connected to each other in such a way that the nozzle part ( 22   a ) of the first stage of the impactor ( 20 ) is simultaneously the bottom plate of said mobility analyzer ( 10 ).

The invention relates to a device for determining the size distributionof aerosol particles as set forth in the preamble of claim 1.

[0001] With tightening environmental regulations, there is an increasingneed for real-time measurement of particle emissions. In particular, theneed for measurement is present in the development of filtering methods,in the research of various combustion processes, as well as inmonitoring processes for actual emissions. For classifying the mobilitysize distribution of aerosol particles, real-time aerosol measurementsapply various analyzers which measure the electric mobility ofparticles, such as differential mobility analyzers (DMA).

[0002]FIG. 1 shows, in a simplified diagram, a mobility analyzer 10according to prior art. The mobility analyzer 10 consists of anelectrically conductive, preferably cylindrical frame part 12, which iscoupled to a first constant potential, typically the ground plane, andthereby acts as an outer electrode. The ends of the frame part 12 areprovided with cover and bottom plates 17 a and 17 b. An inner electrode13 is placed centrally in the frame part 12 and coupled via a powersupply 15 to a second constant potential. The power supply 15 is used toproduce an electric field between the frame part 12 of the device andthe inner electrode 13.

[0003] The cover and bottom plates 17 a and 17 b of the analyzer 10 areequipped with the necessary ducts to implement inlet and outlet pipes18, 14 and 16 as well as the couplings required by the inner electrode13.

[0004] So that particles could be separated on the basis of theirelectric mobility by means of the electric field, the aerosol particlesto be introduced into the analyzer must have an electric charge beforethe analyzer 10. For this reason, the aerosol particles are typicallycharged by a separate charger (not shown in FIG. 1) before the analyzer10. The flow 11 coming from the charger is led via the inlet pipe 18 tothe analyzer. The flow 11 passes through the analyzer 10, primarilyexiting via the outlet pipe 16.

[0005] When entering the electric field between the frame part of theanalyzer 10 and the inner electrode 13, the charged aerosol particles inthe flow 11 are drawn, depending on the sign of the charge, eithertowards the frame part 12 or towards the inner electrode 13. Typically,mobility analyzers are implemented in such a way that interestingaerosol particles are drawn towards the inner electrode 13.

[0006] The rate at which the aerosol particles drift towards theelectrodes depends on the electric mobility of the particles, which isdependent, in a known manner, on e.g. the mass and charge of theparticle.

[0007] Particles having greater electric mobility move faster towardsthe electrode determined by their charge, typically the inner electrode13. As a result of this, the particles with greater electric mobilityhit the inner electrode 13 sooner than particles with smaller electricmobility. As the flow 11 passes towards the bottom plate 17 b of theanalyzer 10, particles with greater electric mobility hit closer to theend of the inner electrode 13 on the side of the cover plate 17 a, andparticles with smaller mobility hit closer to the end of the innerelectrode 13 on the side of the bottom plate 17 b. Those particles whoseelectric mobility is so small that they do not reach the inner electrode13 when moving with the flow 11 through the analyzer, are discharged viathe outlet pipe 16 from the analyzer.

[0008] Depending on the solution to be used, the above-presentedmobility analyzer 10 can be implemented by a number of different ways.In its simplest form, the solution does not apply the outlet pipe 14placed inside the electrode 13 at all, wherein the complete flow 11coming into the analyzer 10 is discharged through orifices in the bottomplate 17 b into the outlet pipe 16. Thus, it is possible to use theanalyzer to remove from the flow 11 the particles which have greaterelectric mobility than a certain value and which are deposited on theinner electrode 13.

[0009] DMA analyzers are typically implemented in such a way that theinner electrode 13 is provided with a narrow slit 19 which is coupled tothe second outlet pipe 14. Thus, particles falling into the slit 19 areabsorbed into the second outlet pipe 14. If the analyzer 10 isconstructed in such a way that the flow 11 to be measured is introducedinto the analyzer 10 at a certain distance from the inner electrode 13,the voltage difference between the frame part 12 of the analyzer 10 andthe inner electrode 13 can be adjusted to determine the mobility rangeof the particles falling into the slit 19. Thus the DMA analyzer can beused to separate particles falling into a certain mobility range fromthe flow 11 under analysis, by guiding them into the second outlet pipe14, wherein particles with greater electric mobility adhere to the innerelectrode 13 before the slit 19 and particles with smaller mobility aredischarged with the flow along the outlet pipe 16.

[0010] Furthermore, it is known to implement the mobility analyzer 10 asa multi-channel differential mobility analyzer, in which the innerelectrode 13 of the mobility analyzer 10 is equipped with severaldetection surfaces which indicate the number of particles hitting them,for example on the basis of the charges transferred by the particleshitting them. The detection surfaces are preferably placed onto thesurface of the inner electrode 13 in such a way that each of themcollects particles falling into a certain mobility range. By monitoringthe signals given by these detection surfaces, it is possible tomeasure, in real time, the electric mobility of particles in the flow 11under analysis and, on the basis of this, to compute the sizedistribution based on the electric mobility diameter of the particles.

[0011] The mobility analyzers are most precise for particles with asmall mass, because the electric mobility of the particles is inverselyproportional to the mass of the particle; that is, the greater the massof the particle, the smaller the mobility of the particle. In typicalparticle measurement environments, the aim is to measure particles whosediameters vary from some tens of nanometres to tens of micrometres. Inthis range, there are typically differences of several orders in themobility of the particles, wherein it is extremely difficult to measurethe whole range simultaneously by one device with a sufficiently highprecision. Electric mobility analyzers are primarily used in fineparticle analyses, in which the mobility analyzers are most precise, dueto the high mobility of the fine particles.

[0012] For measuring primarily larger particles, classification methodsbased on the aerodynamic diameter of the particles are normally used,such as impactors. The electrical low pressure impactor (ELPI) developedby Dekati Ltd provides a solution for real-time measurement of aerosolparticles.

[0013]FIG. 2 shows a cross-sectional view of two upper stages 21 a and21 b of an electrical low pressure impactor 20, and chambers 29 a and 29b related to them. An air flow 11 to be analyzed is sucked by means ofan underpressure produced by a pump (not shown in FIG. 2) into theelectrical low pressure impactor 20 and into the first chamber 29 a ofthe impactor. Each stage comprises a nozzle part 22 a, 22 b, equippedwith orifices, through which the air flow 11 carrying particles flows.Collection surfaces 23 a, 23 b are placed behind the nozzle parts 22 a,22 b. Each collection surface 23 a, 23 b is equipped with at least oneoutlet 25, through which the flow 11 is allowed to flow to the nextchamber or out of the impactor. Insulators 24 a, 24 b, 24 c placedbetween the stages 20 a, 20 b insulate the different stages 20 a, 20 bfrom each other and the first stagel of the impactor from the cover part26.

[0014] The direction of the air flow 11 flowing from the orifices of thenozzle part 22 a, 22 b is abruptly changed when it meets the collectionsurface 23 a, 23 b. The particles carried by the flow 11 and having asufficiently large aerodynamic particle size cannot follow the abruptchange in the direction of the flow, but they hit the collection surface23 a, 23 b, being deposited on the same. When hitting the collectionsurface 23 a, 23 b, the charged particles cause a change in the chargelevel of the collection surface 23 a, 23 b. As the collection surface 23a, 23 b is electrically coupled to said impactor stage 20 a, 20 b whichis, via an electrical coupling 27 a, 27 b further connected to a controlunit 28, the change in the charge level of the collection surface 23 a,23 b is indicated as an electric current which can be detected bysensitive current meters placed in the control unit 28.

[0015] The above-described method makes it possible to classify theparticles selectively according to the size. By selecting, in a knownway, the number and size of orifices in the nozzle part 22 a, 22 b, thedistance between the nozzle part 22 a, 22 b and the collection surface23 a, 23 b, as well as the flow rate to be used, each impactor stage 20a, 20 b can be dimensioned so that only particles larger than a givenparticle size are deposited on the collection surface at each stage. Bydimensioning successive impactor stages in such a way that particleswith different particle sizes are collected on different stages, thecurrents from the different impactor stages 20 a, 20 b measured by thecontrol unit 28 can be used to determine, in real time, the sizedistribution of the particles in the flow 11 to be measured, on thebasis of the aerodynamic diameter.

[0016] As a result of the operating principle of the impactor, theimpactors are most sensitive to particles whose mechanical mobility issmall, i.e. typically particles with a large aerodynamic diameter. Onthe other hand, it is difficult to measure small particles with theimpactors, because, due to the high mechanical mobility of theparticles, high flow rates and abrupt changes in the direction of floware thus required to separate the particles from the flow.

[0017] A problem with the above-presented real-time particlemeasurements of prior art has been their inapplicability formeasurements in a wide range of particle sizes. DMA analyzers aresuitable for the measurement of particles with a small diameter and highelectric mobility, and impactors are suitable for the measurement ofparticles with a very large aerodynamic diameter.

[0018] In certain applications, it is advantageous to carry out themeasurement of the particle size distribution within a wide range ofparticle sizes. If the measurement is made with an electric mobilityanalyzer, the problem is the poor sensitivity of the device for largeparticles, and if an impactor is used, the problem is the detection ofsmall particles with a sufficient precision. Attempts have been made tosolve this problem with prior art devices by carrying out themeasurement in two parts, wherein an electric mobility analyzer 10 isfirst used to measure the size distribution of small particles. Afterthis, the flow obtained from the mobility analyzer 10 through the outletpipe 16 is guided to a separate impactor 20 to determine the sizedistribution of larger particles.

[0019] A problem in the above-described solution of prior art lies inthe joint operation of the two separate measuring devices. Jointmeasurements with other measuring devices are typically not taken intoaccount in the design of the measuring devices, wherein it is difficultto implement the centralized control of the different measuring devices.Furthermore, a problem may be presented by particle losses in thetransfer pipe system between the different measuring devices.

[0020] It is an aim of the present invention to provide a novel devicefor the measurement of the size distribution of aerosol particles toovercome the above-presented problems of the solution of prior art. Thisis achieved by connecting an electric mobility analyzer and an impactorto each other in such a way that the bottom plate of the mobilityanalyzer is simultaneously used as the nozzle part of the first stage ofthe impactor.

[0021] More precisely, the device according to the invention ischaracterized in what will be presented in the characterizing part ofclaim 1. Advantageous embodiments of the invention will be presented inthe dependent claims.

[0022] In the following, the invention will be described in detail withreference to the appended drawings, in which

[0023]FIG. 1 shows an electric mobility analyzer according to prior art,

[0024]FIG. 2 shows an electric impactor according to prior art, and

[0025]FIG. 3 shows a measuring device according to the invention.

[0026]FIGS. 1 and 2 have been discussed above in connection with thedescription of prior art.

[0027]FIG. 3 shows a device 30 for measuring the size distributions ofaerosol particles according to the invention. The device 30 combines amobility analyzer 10 and an impactor 20 into one device. As describedabove in connection with the operation of the mobility analyzer, thesolution of the invention introduces a flow 11 to be analyzed via aninlet pipe 18 into the mobility analyzer 10. In the above-describedmanner, particles with a given electric mobility hit the inner electrode13 of the mobility analyzer 10.

[0028]FIG. 3 shows a solution based on the above-mentioned multi-channelmobility analyzer, in which the inner electrode 13 of the mobilityanalyzer 10 is equipped with several measuring surfaces. However, thesolution of the invention is not limited solely to said analyzersolution, but the mobility analyzer belonging to the device 30 accordingto the invention can also be implemented in another way, for examplewith a solution similar to that shown in FIG. 1, in which particlesfalling into a given mobility range are guided through a slit 19 into asecond outlet pipe 14, the main flow 11 continuing its travel past theopening 19.

[0029] In the device 30 shown in FIG. 3, the flow passed through themobility analyzer 10 is immediately guided into the impactor 20 coupledafter the mobility analyzer 10. The coupling between the mobilityanalyzer 10 and the impactor 20 is preferably implemented so that thebottom plate 17 b of the mobility analyzer 20, shown in FIG. 1, is thenozzle part 22 a of the first stage of the impactor 20, shown in FIG. 2.

[0030] The impactor 20 of FIG. 3 comprises three stages 23 a, 23 b and23 c which are electrically separated by means of insulators 24 a, 24 b,24 c, 24 d from each other and from the mobility analyzer 10.

[0031] The presented solution makes it possible to merge the mobilityanalyzer 10 and the impactor 20 into one compact analysis device 30. Forexample, the devices to be used for the control and calibration of thedevices, such as the means required for adjusting the voltages of themobility analyzer 10, can be centrally placed in a single control unit28. Furthermore, the mobility analyzer 10 and the impactor 20 beingclearly integrated in a single unit, the devices can be easily designedto support each other from the beginning, wherein the calibration andcontrol of the integrated device can be implemented in a considerablysimpler way than in the case of two separate measuring devices.

[0032] The mobility analyzer 10 and the impactor 20 can beadvantageously designed in such a way that the mobility analyzer 10 isused to measure particles smaller than a given mobility-sizedistribution value, and the impactor 20 is used to measure particleslarger than a given aerodynamic diameter value. The above-mentionedvalues can be preferably determined so that the mobility analyzermeasures within the particle size range where it is more accurate thanthe impactor, and the impactor measures within a particle size rangewhere it has a better measurement accuracy than the mobility analyzer.Naturally, the measurement ranges may also be partially overlapping.

[0033] Thanks to the solution of the invention, possible losses in thetransfer pipe between the devices are also eliminated, which increasesthe reliability of the measurement, compared with the solution of twoseparate analyzers placed one after the other.

[0034] The solution of the invention is particularly well suitable formeasurements of size distribution in real time, but it is not, however,solely limited to this solution, because the solution of the inventionis also applicable in so-called integrating measurements (laittaisinnäin vaikka vaikka alkutekstissälukeekin “mittaukseen”), in whichparticles to be measured are deposited in the analyzer for a certainperiod of time. After depositing the particles, the particlesaccumulated during the whole measurement period are measured.

[0035] The solution according to the invention is not limited solely tothe above-described examples, but it may vary within the scope definedby the claims. In particular, the invention is not limited to themobility analyzer and impactor types used in the examples, but thesolution of the invention can be implemented with various mobilityanalyzer and impactor types, known as such.

[0036] Naturally, it will be obvious for anyone skilled in the art thatthe term “bottom plate” of the mobility analyzer, when used in thedescription and in the appended claims, generally refers to that part ofthe mobility analyzer through which the flow exits the analyzer. Saidterm should not be interpreted in a limited sense in such a way that itwould always be the lowermost part of the mobility analyzer. Ifnecessary, the bottom part may also comprise a design different from theplate-like shape, and protruding parts, etc., to implement the device ofthe invention.

1. A device (30) for determining the size distribution of aerosolparticles from a flow (11), characterized in that the device (30)comprises an electric mobility analyzer (10) and an impactor (20),connected to each other in such a way that the nozzle part (22 a) of thefirst stage of the impactor (20) is simultaneously the bottom plate ofsaid mobility analyzer (10), and said mobility analyzer (10) and saidimpactor (20) are designed so that said mobility analyzer (10) collectsparticles smaller than a given mobility diameter value, and saidimpactor (20) collects particles larger than a given aerodynamicdiameter value.
 2. The device (30) according to claim 1, characterizedin that said impactor (20) is an electrical low pressure impactor. 3.The device (30) according to claim 1, characterized in that saidmobility analyzer (10) is a multi-channel differential mobilityanalyzer.
 4. The device (30) according to claim 1, characterized in thatsubstantially the whole flow (11) passing through said mobility analyzer(10), excluding aerosol particles deposited in the mobility analyzer(10), is guided through the nozzle part (35) of the first stage of theimpactor (20).
 5. The device (30) according to claim 1, characterized inthat said mobility diameter value and said aerodynamic diameter valuecorrespond to the same particle size.